1. Field of the Invention
The present invention relates to a visual projection display unit, and more particularly to a polarized light converting device in which a polarized light separator and a polarized light converter are integrated to increase optical efficiency.
2. Description of the Prior Art
A visual projection system such as a projection television and a video projector produce images from a visual display means. The latter may be a specially manufactured small cathode-ray tube (CRT), liquid crystal display (LCD) device, or polymer dispersion liquid crystal display (PDLCD) panel. A visual projection system obtains a large-sized screen from the original image by using a projector lens.
FIG. 1 is a view showing an optical system of a known projection display unit using a conventional polarization beam splitter. As shown in FIG. 1, a prior art projection display unit 10 comprises a light source 11, a polarization beam splitter 12 (hereinafter referred to as PBS) which receives a light beam L projected from light source 11. The PBS separates a reflected S polarized light wave 18 and a transmitted P polarized light wave 19 from the incident light beam L. A plate reflecting mirror 13 then reflects S polarized light wave 18 to proceed in the same direction as P polarized light wave 19 and parallel thereto. A polarized light converter 14 of a half-wavelength plate-converting phase, having a phase difference of λ/2, then transmits S polarized light wave 18, slot hat its phase legs that of P polarized light wave 19 by a half wavelength.
Projection display unit 10 further comprises a condenser 15 for condensing P polarized light wave 19 and S polarized light wave 18 after they are transmitted through the half wavelength plate. Then, a liquid crystal display (LCD) device 16 transmits light projected from condenser 15 to a projector lens unit 17, which projects an incident light L2 onto a screen (not shown) as an enlarged image.
Thus, when a light L projected from light source 11 enters into PBS 12, projection display unit 10 separates L into two polarized light components. Unit 10 transmits a P polarized light wave and reflects an S polarized light wave. The transmitted P polarized light wave enters into the LCD device. At the same time, the reflected S polarized light wave is converted into a P polarized light wave by polarized light converter 14, and then, enters the same LCD device.
FIG. 2 is a view showing a further construction of an optical system of a projection display unit using a conventional polarization separating glass. As shown in FIG. 2, a projection display unit 20 using a conventional polarization separating glass comprises a light source 21 providing a light beam L, a polarization separating medium 22 which separates reflected S polarized light wave 28 and transmitted P polarized light wave 29 from light L projected from the light source 21. A plate reflecting mirror 23 reflects P polarized light wave 29 in the same direction as S polarized light wave 28 and parallel thereto. A polarized light converter 24 of a half-wavelength plate-converting phase transmits S polarized light wave 18, which has changed its direction by the action of reflecting mirror 23 and has a phase difference of λ/2.
Projection display unit 20 further comprises a LCD device 25 for transmitting the P polarized light wave 29 and the S polarized light wave. A projector lens unit 27 projects an incident light L1 onto a screen(not shown) as an enlarged image.
Thus, when a light projected from the light source enters the polarization separating medium, the projection display unit separates two polarized light components. It transmits a P polarized light wave and reflects a S polarized light wave. The reflected S polarized light wave enters the LCD device. The direction of the transmitted P polarized light wave is changed by the reflecting mirror, and the transmitted P polarized light wave is converted into a S polarized light wave by polarized light converter 24. Finally, it enters the LCD device.
However, in each instance, the projection display unit is subject to a decrease of 50% in optical efficiency; also using an expensive polarized light converter causes a substantial increase in cost. Further, since a small polarized light separator and a polarized light converter are connected and used at a right angle to the light direction, in order to ensure spatial uniformity of intensity of light, the necessary manufacture and assembling processes are complex and difficult.
It is known that there exists a particular angle of incidence, called the Brewster Angle, θB, at which there is zero reflection and complete refraction of the parallel components of the electric field vectors in light incident on a glass or other dielectric surface. The light reflected from the glass, for this angle, is fully polarized and its plane of vibration is normal to the plane of incidence. In this configuration, the reflected and refracted rays of light are perpendicular to one another. Therefore, the angle of reflection (θB) and the angle of refraction (θr) have a relationship such that θB+θr=90 degrees. This leads to derivation of the Brewster Angle as       θ    B    +            tan              -        1              ⁢                  N        bm                    N        o            where Nbm=refractive index of dielectric medium and No=refractive index of air. See, e.g., Holiday, Resnick, & Walker, Fundamentals of Physics 1017-18 (4th ed. 1993).